1999 GMC Savanna 3500. Engine Size : 5.7 Question Here: Master cylinder leaks from reservoir replaced 3 times plus lids no sign of any other leaks. Brake pedal feels low replaced all brake parts?
Answer: You probably still have air in the brake lines somewhere. They are difficult to bleed at times, especially if you don't have anyone to help you. Also, if you have anti lock brakes on the van, there is a special procedure to bleed the ABS unit, that requires a scanner. It can be done without a scanner, but is a long process. The master cylinder should have been bled on the bench before installing to the vehicle, and then again once installed. If you have not done this, you can try to bleed the master cylinder by removing the lines from it and have someone slowly depress the brake pedal while you hold your fingers over the holes.
Once the fluid starts to squirt out around your fingers, watch for air bubbles in the fluid to be gone and then replace the lines. Gravity bleed the front and rear brakes, preferably running an entire brake fluid bottle through the system. Keep topping off the master as necessary. Tighten all bleeders, then go through the regular bleeding procedure starting at the right rear, then left rear, then right front, then left front. If that doesn't help, you either have a problem still or need the ABS unit bled using a scanner.